Congenital heart malformations, the most common human birth defects, occur in nearly 1% of the population worldwide. Typically, they result from abnormal lineage decisions of early progenitors or disruptions in patterning, both leading to morphogenetic defects. In addition, heart disease is the leading killer of adults in the U.S. Deciphering the secrets of heart formation might lead to novel approaches to repair or regenerate damaged heart muscle by harnessing the potential of stem cell biology. The overall goal of this PPG is to focus the efforts of multiple investigators to dissect the signaling and transcriptional pathways that dictate early decisions of cardiac differentiation in unique regions of the heart and to reveal the mechanisms that guide patterning events during cardiogenesis. In a coordinated fashion, we will test the hypothesis that specific signaling and transcriptional networks govern the decisions of distinct regions of myocardium and result in specific sublineage decisions and patterning of functional regions of the heart. The project and core leaders have been collaborating for several years, and the proposal arises from mutual and complementary interests and approaches. Project 1 will determine the mechanisms by which canonical Wnt/?-catenin signaling and its downstream transcriptional events promote cardiac proliferation and differentiation in specific domains of the mouse heart in vivo. This knowledge will be complemented by and used to manipulate embryonic stem (ES) cells into the cardiomyocyte fate. Project 2 will explore the mechanisms underlying the differentiation of cardiomyocytes in the septal region and at the atrioventricular boundary, particularly as they relate to the function of key transcription factors such as Tbx5 and Nkx2.5 during patterning of the mouse heart; this will be done in vivo and in ES cells, as in Project 1. Project 3 will determine the mechanism by which Bmp4 patterns the outflow tract myocardium to influence valve formation at the ventriculo-arterial boundary in mice and will use a Mef2-dependent valvular enhancer to dissect the signaling networks leading to domain-specific gene expression. This project overlaps with the valve interests of Projects 2 and will also integrate the Wnt signals involved in valvulogenesis and possibly Mef2c activation. Three scientific cores will support the proposed experiments, as well as an administrative core. The synergistic and mutually reinforcing projects and cores in this proposal combine expertise in mouse genetics, stem cell biology, cell biology, developmental biology and genomics to tackle the fundamental problem of patterning of the developing heart, particularly as it relates to disease.